With non-stock (assuming not jailbroken but just a totally different operating system) I think (I might be wrong... I should know for sure but I awkwardly don't) you aren't even allowed to use Google Play Services at all?

You are allowed to use Google services. There is even an alternative called microG which is compatible with apps requiring Google serivices but it sends "fake" data to Google.

back in the day it was not allowed, and then they allowed it somewhat begrudgingly. this was like 10y ago though.

no idea what the situation is now, but i wouldn't consider a phone that doesn't get those patches (project mainline) on time to be a serious option.